1. Field of the Invention
Embodiments of the invention relate to the field of wireless communication networks, more specifically to the field of heterogeneous networks comprising femtocells. More specifically, embodiments concern a method for assigning frequency subbands to a plurality of interfering nodes in a wireless communication network, to a controller for a wireless communication network, and to a wireless communication system including such a controller.
2. Description of the Related Art
Heterogeneous networks promise high system performance in terms of capacity and coverage. A femtocell is one of the important parts of such networks. In networks where femtocells are deployed densely, interference mitigation between such femtocells becomes crucial in order to provide for a desired quality of service (QoS). In wireless networks, data traffic increases day by day and mobile operators face difficulties in satisfying users' demands. One solution to this problem is the introduction of a femtocell access point (FAP) also known as home evolved nodeB (HeNB). These access points or nodes are small base stations deployed by users and are mostly used for indoor environments. FIG. 1 is a schematic representation of a network cell 100 comprising a base station 102. In FIG. 1 an indoor environment 104 is schematically illustrated which lies within the cell 100. The indoor environment 104, for example, includes a first room 1041 and a second room 1042. In each room 1041 and 1042 a user deployed femtocell access point or home evolved nodeB is indicated by the reference signs HeNB-1 and HeNB-2. Within each room 1041 and 1042 a user equipment FUE-1 and FUE-2 is positioned. Further, within the cell 100 one mobile user equipment MUE is shown. The user equipment FUE-1 positioned in the first room 1041 of the indoor environment 104 directly communicates with the base station 102 as is indicated by arrow 1. The mobile user equipment MUE provided outside the environment 104 and inside the cell 100 communicates with the femtocell access point HeNB-1, as is indicated by arrow 2. In the second room 1042 of the indoor environment 104 a further user equipment FUE-2 is provided which also communicates with the femtocell access point HeNB-1 in the first room 1041 of the indoor environment 104.
The main advantage of the HeNBs is the significant improvement in indoor coverage and capacity that cannot be achieved by using macrocells only, as it is for example described by H. Claussen, “Performance of Macro- and Co-Channel Femtocells in a Hierarchical Cell Structure,” in Proc. Of the 18th IEEE International Symposium on Personal, Indoor and Mobile Radio Communications (PIMRC), Athens, Greece, Sep. 3-7 2007, pp. 1-5, and by Z. Bharucha, H. Haas, A. Saul, and G. Auer, “Throughput Enhancement through Femto-Cell Deployment,” European Transactions on Telecommunications, vol. 21, no 4, pp. 469-477, Mar. 31 2010. Since the coverage area of an HeNB is small, the available spectrum can be reused more often. Also, as the indoor users are served by HeNBs, the traffic load of the macrocell 100 decreases, which is another advantage of the femtocell deployment by operators, as is also described by V. Chandrasekhar, J. Andrews, and A. Gatherer, “Femtocell Networks: A Survey,” IEEE Communications Magazine, vol. 46, no. 9, pp. 59-67, 2008.
However, the deployment of femtocells also comes with some problems. Among such problems interference between femtocells (co-channel interference) needs more attention, especially in networks where femtocells are densely deployed, such as a network of a company, a shopping mall, etc. Unlike macrocells, femtocells are placed by end users, so that frequency planning is not possible. In addition, there can be situations where two femtocells are deployed very close to each other, and in such situations user equipments (UEs) face a high interference from neighboring femtocells, and these UEs probably go into outage. In FIG. 1, such an interference situation is schematically shown between the user equipment FEU-2 in the second room 1042 and the HeNB-1 in the first room 1041 of the indoor environment. Thus, the deployment of femtocells allows for an increase in coverage, an increase in data rate, however, this comes with an increase in interference. Thus, conventional approaches have the problem that user experience in femtocell networks cannot be maintained at an acceptable level.
One known solution to this problem is applying a resource partitioning approach. In accordance with such an approach, neighbors interfering with each other use different subbands, which are also called priority subbands having a maximum transition power. The rest of the subbands, the so called secondary subbands, are not used or are used with a power control so as not to interfere with the priority band of the neighboring femtocell. FIG. 2 illustrates the approach of interference mitigation by resource partitioning. FIG. 2(A) shows an example of three cells A, B and C adjacent to each other, each cell comprising a base station eNBA, eNBB and eNBC. The first cell A uses a first resource 1, for example a first frequency band within an available frequency range. The second cell B uses a second resource 2, for example a second frequency band, and cell C uses a third resource 3, for example a third frequency subband. FIG. 2(B) illustrates how interference mitigation by resource partitioning is achieved, namely by selecting the subbands 1 to 3 in such a way that the cells A to C use non-overlapping priority subbands within the available frequency range F. In FIG. 2(B), an example is shown comprising the three neighboring cells A, B and C in which a first frequency band 1 is used by the cell A, whereas the remaining subbands, the secondary subbands ii and iii are either not used at all or with reduced power when compared to the priority subband 1. In a similar manner cell B has as priority subband the subband 2 and the remaining, secondary subbands i and iii are either not used or with a reduced power. The same is true for cell C using the third subband 3, wherein the first and second secondary subbands i and ii are not used or with a reduced power. As can be seen from FIG. 2, maximizing the system capacity and maintaining an acceptable user experience to all users may be contradicting goals. The interference management by resource partitioning allows a cell center user to use all resources with reduced power, while cell-edge users are assigned priority bands, where they may transmit with full power.
Thus, the UEs to which a priority subband is allocated face less interference and enjoy higher capacity values. However, resource partitioning decreases the resource efficiency of the network. The more bandwidth is assigned as a secondary band, the less resources are used with maximum available power. For macrocell networks a variety of resource partitioning approaches are known. In such networks neighbors of a base station are known a priori including the locations and cell IDs. Depending on the number of neighbors and the locations the total frequency band is divided into orthogonal regions and each base station uses one of these regions as its priority subband.
Using such an approach may be difficult in femtocell networks and the above described resource partitioning approach may not be applicable to such networks easily. FIG. 3 shows a schematic representation for illustrating how priority subbands using the resource partitioning approach may be assigned in a femtocell network. FIG. 3(A) illustrates schematically an indoor environment 104 having a plurality of rooms 1041 to 10410 in which in rooms 1041 to 1046 respective femtocell access points A to F are installed or deployed by a user. For example, in FIG. 3 there may be three FAPs (femtocell access points), A, B and C, at the beginning. In this case, resource partitioning as shown in FIG. 2 can be used. However, after a certain time additional FAPs, e.g. D, E and F, may enter the network. As can be seen, the femtocell access points (HeNBs) are provided in neighboring rooms 1041 to 1045 while femto access point F is arranged in room 1046, distant from the remaining femtocell access points. The arrows in FIG. 3(A) illustrate possible interference paths between the respective femtocells, and as can be seen, it is assumed that cell A may interfere with cells B to E but not with cell F. Cell B in room 1042 is assumed to interfere with cells A and C, but not with cells D to F. Cell C in room 1043 is assumed to interfere with cells A and B but not with cells D to F. Cells D and E are assumed to interfere only with cell A while cell F, as mentioned above, is further away from the remaining cells so that no interference is assumed. Applying the above described approach of resource partitioning yields a frequency subband distribution as it is shown in FIG. 3(B), which is similar to the one shown in FIG. 2(A) in that the three available subbands within the frequency range are distributed among cells A, B and C, however, this does not cover cells D, E and F as is indicated by the question marks in FIG. 3(B). Thus, FIG. 3 shows the necessity of dynamic resource partitioning. Dynamic resource partitioning may be done in a centralized way or in a distributed way.
In a distributed approach, each base station determines the resources used by itself. Distributed resource partitioning methods in macro and femto networks are described e.g. by:
Y.-Y. Li, M. Macuha, E. Sousa, T. Sato, and M. Nanri, “Cognitive interference management in 3G femtocells,” in Personal, Indoor and Mobile Radio Communications, 2009 IEEE 20th International Symposium on, Sep. 13-16 2009, pp. 1118-1122,
J. Ling, D. Chizhik, and R. Valenzuela, “On Resource Allocation in Dense Femto-deployments,” in Microwaves, Communications, Antennas and Electronics Systems, 2009, COMCAS 2009, IEEE International Conference on, Nov. 9-11 2009, pp. 1-6,
J. Ellenbeck, C. Hartmann, and L. Berlemann, “Decentralized Inter-Cell Interference Coordination by Autonomous Spectral Reuse Decisions,” in Wireless Conference, 2008, EW 2008, 14 European, Jun. 22-25 2008, pp. 1-7, and
C. Lee, J.-H Huang, and L.-C. Wang, “Distributed Channel Selection Principles for Femtocells with Two-Tier Interference,” in Vehicular Technology Conference (VTC 2010—Spring), 2010 IEEE 71st, May 16-19 2010, pp. 1-5.
In accordance with such known methods, each (H)eNB uses only a predefined number of subbands for transmission. Changing interference conditions are neither recognized nor handled. Another drawback of such approaches is that the resources to be used are determined by listening to the environment and there is no coordination between the neighboring (H)eNBs. Thus, in accordance with the distributed approach the nodes or frequency access points determine the resource they will use, however, only a predefined number of resources per node or (H)eNB is used which results in a low subband usage and a convergence problem.
In the central approach, on the other hand, there is a central controller which takes interference information from all nodes or (H)eNBs and assigns the priority subbands to each (H)eNB according to these feedbacks. Since the priority bands are assigned centrally, a more efficient resource utilization may be achieved. The central approach provides for a quick convergence, is effective for networks where cells are densely deployed, however, needs a central controller such as HeNB-GW (GW=gate way).
The most common approach used in central resource assignment is the so called graph theory where the interference relation between cells is mapped into a graph (interference graph). FIG. 4 shows an example of an approach for resource assignment using the graph theory. FIG. 4(A) shows a schematic representation of an indoor environment comprising six rooms 1041, 1042, 1043, 1044, 1045 and 1046. In this indoor environment 104 rooms 1041 to 1043 are provided with (H)eNBs A to C. The circles around the nodes A to C show their range. As can be seen the ranges overlap. Further, in accordance with the central approach, a central controller 106 is provided that gathers from the respective nodes A to C respective interference information. The central controller 106 generates an interference graph that is depicted in FIG. 4(B), wherein interfering neighbors are for example defined according to a predefined parameter threshold (e.g. SINR=Signal to Interference and Noise Ratio). In the interference graph 108 the nodes A to C correspond to a respective cell (indicated by the circles in FIG. 4(A)), and the edges connecting two nodes represent the interference between the respective cells. Since the cells or ranges of the nodes A to C are intersecting and overlapping the interference graph 108 shows that each nodes A to C interferes with its neighboring node.
Once the interference graph, like the interference graph in FIG. 4(B), is generated priority subbands are assigned according to the constraints in the interference graph. This is generally done by applying graph coloring algorithms which have a low complexity. Resource assignment using the graph coloring algorithms for a macrocell networks is described by:
Chang, Z. Tao, J. Zhang, and C.-C. Kuo, “A Graph Approach to Dynamic Fractional Frequency Reuse (FFR) in Multi-Cell OFDMA Networks,” in Communications, 2009, ICC '09, IEEE International Conference on, Jun. 14-18, 2009, pp. 1-6,
M. C. Necker, “Integrated scheduling and interferences coordination in cellular OFDMA networks,” in Broadband Communications, Networks and Systems, 2007, BROADNETS 2007, Fourth International Conference on, Sep. 10-14 2007, pp. 559-566, and
“A Graph-Based Scheme for Distributed Interference Coordination in Cellular OFDMA Networks,” in Vehicular Technology Conference, 2008, VTC Spring 2008, IEEE, May 11-14 2008, pp. 713-718.
The interference graph is constructed on the basis of UEs. Since interference conditions of UEs change more frequently, such interference graphs should be updated more frequently which causes a high amount of signalling. Also, in Chang, Z. Tao, J. Zhang, and C.-C. Kuo, “A Graph Approach to Dynamic Fractional Frequency Reuse (FFR) in Multi-Cell OFDMA Networks,” in Communications, 2009, ICC '09, IEEE International Conference on, Jun. 14-18, 2009, pp. 1-6, subbands usage efficiency of the whole network is not deeply investigated. On the other hand, in “A Graph-Based Scheme for Distributed Interference Coordination in Cellular OFDMA Networks,” in Vehicular Technology Conference, 2008, VTC Spring 2008, IEEE, May 11-14 2008, pp. 713-718 UEs, are colored with one or more colors by a central controller and then each base station allocates its serving UEs one or more resource partitions among the assigned color set of UEs in a way to increase resource allocation. Apart from graph coloring, in D. López Pérez, G. de la Roche, A. Valcarce, A. Jüttner, and J. Zhang, “Interference avoidance and dynamic frequency planning for wimax femtocells networks,” in Communication Systems, 2008, ICCS 2008, 11th IEEE Singapore International Conference on, Nov. 19-21 2008, pp. 1579-1584, a central entity assigns the resources using an optimization function to minimize the overall network interference. In this method, the amount of resources assigned to (H)eNBs is estimated in accordance with the traffic demands of each (H)eNB instead of the interference conditions. Therefore, under high traffic load situations where all (H)eNBs necessitate large bandwidths, this approach will fail to assign an interference-free subband to a cell edge user.
Thus, the above described conventional approaches for assigning respective subbands to base stations are not applicable to femtocell networks and are disadvantageous as they do not exploit the complete possible frequency space that may be available and that is needed for effectively assigning priority subbands in a dynamic environment like in a femtocell network. Rather, all conventional approaches dealing with the problem of assigning priority subbands simply select one of a number of possible subbands, in general randomly, so that due to the non-used subbands a decrease of throughput is experienced. The approach described by M. C. Necker, “Integrated scheduling and interferences coordination in cellular OFDMA networks,” in Broadband Communications, Networks and Systems, 2007, BROADNETS 2007, Fourth International Conference on, Sep. 10-14 2007, pp. 559-566, deals with macrocells and is not applicable to femtocell networks because each base station utilizes its resource among the sectors after the subbands were allocated to the user equipments. However, in a femtocell network the HeNB has only one sector so that this approach would not improve the performance as it does in macrocell networks.